Pregnancy is a normal process that begins with conception of a fetus in the woman's body, continues through fetal growth and development, delivery of the infant, and ends with a return of the body to a fully normal state approximately six weeks after birth. Pregnancy causes physiological changes in the mother's bodily functions to accommodate growth and development of the fetus. For the fetus, pregnancy is a time of dependency on the mother for nutrients and, thus, potential transmission across the placenta of everything to which the mother is exposed. Although most women have healthy and normal pregnancies, some women have complications leading to an adverse outcome for the mother or the fetus.
The average biological length of human gestation, from conception to delivery, is 266 days. Because of the difficulty in assessing the exact date of conception, the clinical length of pregnancy is considered to be 280 days, or 40 weeks, calculated from the last menstrual period before the cessation of menses, or menstrual flow. This calculation assumes that ovulation occurs 14 days after the last menstrual period. Ultrasound or other tools can establish gestational age when menses are irregular or unsure. Gestation is divided into trimesters of approximately 13 weeks, but most physicians generally describe pregnancy by the number of completed weeks.
The odds that the baby will be a boy are slightly greater than 50 percent. Worldwide, 106 boys are born for every 100 girls. But because boys have a higher infant mortality rate, girls soon outnumber boys.
After the ovum, or egg, is fertilized by a sperm, the fertilized ovum becomes implanted in the uterus.
Most fetal development, with the exception of such complex functions as brain development, occurs in the first trimester. The heart begins to beat after four weeks. By eight weeks, the eyes, ears, nose, mouth, fingers, and toes are easily recognizable, and male and female reproductive systems have differentiated.
The mother and fetus are physiologically connected via the placenta, a complex organ that filters oxygen and nourishment from the mother's blood to the baby via the umbilical cord. It also removes carbon dioxide and other waste products from the fetus to the mother.
By 12 weeks, all of the recognizable organs have developed. During this period the fetus is most vulnerable to potential teratogenic, or birth defect-inducing, agents, such as drugs, radiation, and viruses. Drugs ingested by the mother during the first weeks can be of particular harm, demonstrated in Europe in the late 1950s when the drug thalidomide, commonly administered as a sedative, was found to be associated with limb deformities.
During the second trimester, thin-walled skin develops, organs begin to function, blood begins to form in the bone marrow, scalp hair appears, subcutaneous fat increases, and bones begin to harden. Although the fetus starts to move in the first trimester, it is not until about 20 weeks gestation that the mother begins to perceive the movements, the onset of which is called "quickening."
The majority of fetal weight gain occurs in the third trimester. Earlobes begin to develop cartilage, testes begin to descend into the scrotum, nails begin to grow over the tips of the digits, and creases develop over the soles of the feet. In addition, the fetus begins to demonstrate coordinated patterns of behavior that are similar to the cycles of sleep and activity of a newborn.